Mercury is the closest planet to the sun. Spacecraft data indicate the planet has virtually no atmosphere and is densely cratered. Basaltic lava flows have also been identified. A metallic core that accounts for about three‐quarters of the planet's mass has been offered as an explanation for the planet's high density of 5.4 g/cm3. Mercury also has a weak dipolar magnetic field. The lack of any active volcanic, tectonic, or erosional processes suggests Mercury is a “dead” planet.